Advances in polymerization and catalysis have resulted in the capability to produce many new polymers having improved physical and chemical properties useful in a wide variety of superior products and applications. With the development of new catalysts the choice of polymerization-type (solution, slurry, high pressure or gas phase) for producing a particular polymer has been greatly expanded. Also, advances in polymerization technology have provided more efficient, highly productive and economically enhanced processes. Especially illustrative of these advances is the development of technology utilizing bulky ligand metallocene-type catalyst systems. Regardless of these technological advances in the polyolefin industry, common problems, as well as new challenges associated with process operability still exist. For example, the tendency for a gas phase or slurry phase process to foul and/or sheet remains a challenge.
For example, in a continuous slurry process fouling on the walls of the reactor, which act as a heat transfer surface, can result in many operability problems. Poor heat transfer during polymerization can result in polymer particles adhering to the walls of the reactor. These polymer particles can continue to polymerize on the walls and can result in a premature reactor shutdown. Also, depending on the reactor conditions, some of the polymer may dissolve in the reactor diluent and redeposit on for example the metal heat exchanger surfaces.
In a typical continuous gas phase process, a recycle system is employed for many reasons including the removal of heat generated in the process by the polymerization. Fouling, sheeting and/or static generation in a continuous gas phase process can lead to the ineffective operation of various reactor systems. For example, the cooling mechanism of the recycle system, the temperature probes utilized for process control and the distributor plate, if affected, can lead to an early reactor shutdown.
Evidence of, and solutions to, various process operability problems have been addressed by many in the art. For example, U.S. Pat. Nos. 4,792,592, 4,803,251, 4,855,370 and 5,391,657 all discuss techniques for reducing static generation in a polymerization process by introducing to the process for example, water, alcohols, ketones, and/or inorganic chemical additives; PCT publication WO 97/14721 published Apr. 24, 1997 discusses the suppression of fines that can cause sheeting by adding an inert hydrocarbon to the reactor; U.S. Pat. No. 5,627,243 discusses a new type of distributor plate for use in fluidized bed gas phase reactors; PCT publication WO 96/08520 discusses avoiding the introduction of a scavenger into the reactor; U.S. Pat. No. 5,461,123 discusses using sound waves to reduce sheeting; U.S. Pat. No. 5,066,736 and EP-A1 0 549 252 discuss the introduction of an activity retarder to the reactor to reduce agglomerates; U.S. Pat. No. 5,610,244 relates to feeding make-up monomer directly into the reactor above the bed to avoid fouling and improve polymer quality; U.S. Pat. No. 5,126,414 discusses including an oligomer removal system for reducing distributor plate fouling and providing for polymers free of gels; EP-A1 0 453 116 published Oct. 23, 1991 discusses the introduction of antistatic agents to the reactor for reducing the amount of sheets and agglomerates; U.S. Pat. No. 4,012,574 discusses adding a surface-active compound, a perfluorocarbon group, to the reactor to reduce fouling; U.S. Pat. No. 5,026,795 discusses the addition of an antistatic agent with a liquid carrier to the polymerization zone in the reactor; U.S. Pat. No. 5,410,002 discusses using a conventional Ziegler-Natta titanium/magnesium supported catalyst system where a selection of antistatic agents are added directly to the reactor to reduce fouling; U.S. Pat. Nos. 5,034,480 and 5,034,481 discuss a reaction product of a conventional Ziegler-Natta titanium catalyst with an antistat to produce ultrahigh molecular weight ethylene polymers; U.S. Pat. No. 3,082,198 discusses introducing an amount of a carboxylic acid dependent on the quantity of water in a process for polymerizing ethylene using a titanium/aluminum organometallic catalysts in a hydrocarbon liquid medium; and U.S. Pat. No. 3,919,185 describes a slurry process using a nonpolar hydrocarbon diluent using a conventional Ziegler-Natta-type or Phillips-type catalyst and a polyvalent metal salt of an organic acid having a molecular weight of at least 300.
There are various other known methods for improving operability including coating the polymerization equipment, for example, treating the walls of a reactor using chromium compounds as described in U.S. Pat. Nos. 4,532,311 and 4,876,320; injecting various agents into the process, for example PCT Publication WO 97/46599 published Dec. 11, 1997 discusses feeding into a lean zone in a polymerization reactor an unsupported, soluble metallocene-type catalyst system and injecting antifoulants or antistatic agents into the reactor; controlling the polymerization rate, particularly on start-up; and reconfiguring the reactor design.
Others in the art to improve process operability have discussed modifying the catalyst system by preparing the catalyst system in different ways. For example, methods in the art include combining the catalyst system components in a particular order; manipulating the ratio of the various catalyst system components; varying the contact time and/or temperature when combining the components of a catalyst system; or simply adding various compounds to the catalyst system. These techniques or combinations thereof are discussed in the literature. Especially illustrative in the art is the preparation procedures and methods for producing bulky ligand metallocene-type catalyst systems, more particularly supported bulky ligand metallocene-type catalyst systems with reduced tendencies for fouling and better operability. Examples of these include: WO 96/11961 published Apr. 26, 1996 discusses as a component of a supported catalyst system an antistatic agent for reducing fouling and sheeting in a gas, slurry or liquid pool polymerization process; U.S. Pat. No. 5,283,218 is directed towards the prepolymerization of a metallocene catalyst; U.S. Pat. Nos. 5,332,706 and 5,473,028 have resorted to a particular technique for forming a catalyst by incipient impregnation; U.S. Pat. Nos. 5,427,991 and 5,643,847 describe the chemical bonding of non-coordinating anionic activators to supports; U.S. Pat. No. 5,492,975 discusses polymer bound metallocene-type catalyst systems; U.S. Pat. No. 5,661,095 discusses supporting a metallocene-type catalyst on a copolymer of an olefin and an unsaturated silane; PCT publication WO 97/06186 published Feb. 20, 1997 teaches removing inorganic and organic impurities after formation of the metallocene-type catalyst itself; PCT publication WO 97/15602 published May 1, 1997 discusses readily supportable metal complexes; PCT publication WO 97/27224 published Jul. 31, 1997 relates to forming a supported transition metal compound in the presence of an unsaturated organic compound having at least one terminal double bond; and EP-A2-811 638 discusses using a metallocene catalyst and an activating cocatalyst in a polymerization process in the presence of a nitrogen containing antistatic agent.
While all these possible solutions might reduce the level of fouling or sheeting somewhat, some are expensive to employ and/or may not reduce fouling and sheeting to a level sufficient to successfully operate a continuous process, particularly a commercial or large-scale process.
Thus, it would be advantageous to have a polymerization process capable of operating continuously with enhanced reactor operability and at the same time produce new and improved polymers. It would also be highly beneficial to have a continuously operating polymerization process having more stable catalyst productivities, reduced fouling/sheeting tendencies and increased duration of operation.